1. Field of the Invention The present invention relates generally to electrical power generating devices and, more specifically, to a power generating device which uses superconducting materials to deflect a magnetic field relative to conductive coils and thereby generate electrical power.
2. Description of the Related Art
An electromagnetic force or voltage is induced in a conducting loop or coil when there is relative motion between the conducting loop and a magnetic field. Thus, the production of electric power by electromagnetic means requires that a conductor(s) be in a changing magnetic field. In an arrangement where the poles of the magnetic field are in a vertical plane, and the conductor is in a horizontal plane in the middle of the field, there are three possibilities for producing an electromagnetic force in the conductor: (1) move the conductor sideways, left and right; (2) move the field sideways, left and right; and (3) alternate the flux density of the field through the conductor.
Traditional electromagnetic generators use mechanical energy to move either the field or the conductor. Power standards regarding voltage and frequency are already established, so generator design developments have so far dealt with the quantity of conductors, relative velocity, magnetic flux density, quantity of magnetic poles, etc.
Generally speaking, known electromagnetic power generators are rotary devices and, generally, the design thereof must address the problems of power transmission across a moving interface through brushes, as well as monitoring and maintaining other moving parts such as rotor bearings and output shaft, etc.
Recent developments in the field of superconductivity have led to dramatic increases in the temperature at which superconductive materials achieve a superconducting state. Thus, as a result of these improvements, generally in the form of materials which become superconductive at higher temperatures, it has only recently become of interest to consider practical applications of superconductivity.
Superconductive materials today are generally known as either Type I or Type II. Type I superconductors are primarily the elemental type such as lead and tin, and will exclude an external magnetic field completely when they transition to the superconducting state, but only up to some limit. Type II superconductors exclude completely much weaker magnetic fields than Type I superconductors. As the magnetic field increases in intensity, the field begins to penetrate through the superconductor, but only in small localized "pin holes" or "pins". As the magnetic field increases in intensity, the quantity and density of distribution of these pins also increases.
The Meissner effect is the expulsion of magnetic flux from the interior of a piece of superconducting material as the material undergoes the transition to the superconducting phase. Under controlled conditions, the Meissner effect is reversible in the presence of a magnetic field. In the presence of a magnetic field, with the temperature of the superconducting material above the critical temperature, the field freely penetrates the superconductive material. As the temperature drops below the critical temperature, the superconductive material transitions into the superconducting state and expels the magnetic field. In the foregoing, some work is involved which results in heat being generated. Assuming conductive heat transfer to a heat sink, such as a cryogenic fluid, the heat is removed almost as quickly as it is generated. The greater the density of the flux being moved, the greater will be the heat generated. With a constant heat removal rate, the expulsion rate would likely vary from fast for low magnetic field density to slow at high magnetic field density.
As the temperature rises above the critical temperature, the superconducting material transitions back to the normal state and the magnetic field returns to penetrate the superconducting material volume. Similarly, the superconducting material responds to changes in the magnetic field strength and conducted current, if the temperature is maintained below the critical temperature. For Type I materials, the transition in a zero field at critical temperature is of second order, while the transition in the presence of a field is of first order since there is a discontinuous change in the thermodynamic state of the system and an associated latent heat.
For Type II materials, the transition from the superconducting state to normal with increasing magnetic field strength is not discontinuous in a first order transition at the critical field. Instead, there is an increase in flux penetration which proceeds in discrete jumps starting as the critical field strength is reached. Since the partial flux penetration reduces the diamagnetic energy required to hold the field out, the critical field can be much greater than the thermodynamic critical field.
Generally, the typical Type II superconducting material exhibits a smoother increase in resistance as the critical current density is approached. The effect of increased resistance will be to produce more heat, which in turn may be sufficient to cause the superconducting structure to exceed the critical temperature. A number of Type II superconducting materials are described in Physica 148B (1987), for example, article 224-227 "Superconductive Properties of Single Crystal YBa.sub.2 Cu.sub.3 O.sub.x " by Yasuhiro Iye, et al.
The stability of the superconducting state in the materials discovered so far is relative to three conditions. First, and the primary condition (so far as we know) is that the temperature must be at or below some critical value. Many materials have been produced experimentally and investigated that have a critical temperature above the boiling point of liquid nitrogen (77.degree. K.). One of the more recently discovered materials has a critical temperature of 125.degree. K.
Secondly, in the superconducting state there is a limit to how much current can be conducted. If the conducted current (current density) exceeds some maximum limit, then the superconducting state rapidly reverts to the normal state. The limit is a variable dependent on the temperature of the material and the magnitude of any externally imposed magnetic field. Thirdly, materials in the superconducting state can withstand an externally imposed magnetic field, but as with conducted current, there is a maximum limit. This limit is variable, dependent on temperature and current.
Materials in a superconducting state can withstand greater externally imposed magnetic fields and support greater current densities as the temperature of the material is reduced. Transition time as the material switches from the normal state to the superconducting state, and visa versa, can be very brief.
To date, superconductivity research has been directed to the current-carrying capabilities and transition temperatures of the materials. Few practical applications of superconductivity technology have been attempted.